How can I create a water material in Cycles?
It is very important to add some small scale displacement to the water surface. This small scale detail is present even in closed spaces due to minor drafts of air, especially in large pools of water. My personal experience is that for best results you can use a combination of animated voronoi textures, like perlin noise does.

This small detail makes it possible to have areas of water that are slightly more refractive or reflective than others. To separate the refraction and reflection parts, you use a standard fresnel factor connected to a mix shader node. 

This is my example shader:

(play with index of refraction to achieve a different ratio of refraction to reflection depending on the angle of viewing)



And this is the resulting water:



Of course on indoor scenes you will need many samples or clever lighting to avoid too many fireflies.